Digital communication systems conventionally use analog circuits near transmit and receive ports and use digital circuits elsewhere. In other words, transmitters convert digital modulated signals to analog signals which are subsequently processed by analog mixing, analog filtering, and amplifying before a communication signal is actually transmitted. Likewise receivers perform certain analog amplifying, analog filtering, and analog mixing prior to converting a received communication signal into a digital signal from which conveyed data are extracted.
Likewise, conventional digital communication systems convey a data stream via quadrature-phase modulation, wherein the data stream is modulated into a complex signal that has orthogonal signal components. Accordingly, in a conventional digital communication system, the orthogonal signal components are often processed separately by separate analog components positioned near transmit and receive ports.
The processing of separate orthogonal signal components by separate analog components causes a long-recognized problem. In particular, errors result when the orthogonal signal components are not truly orthogonal. For the typical situation, this occurs when in-phase (I) signals and quadrature-phase (Q) signals are not precisely 90.degree. apart. Errors also result when the peak amplitudes of orthogonal signal components are not precisely equal and when carrier leakage occurs. Carrier leakage results when the carrier does not precisely equal zero for a zero modulating waveform.
Prior art solutions to this quadrature imbalance problem include the use of well-matched analog components and the use of analog components that are trimmed with trim potentiometers and the like. However, this solution is undesirable because it leads to the use of expensive components, expensive labor costs during manufacture, and reliability problems as components drift over time and temperature.
Another prior art solution to this quadrature imbalance problem relies on adaptive equalizers and other adaptive circuits located in the receiver. While this solution has been adequate for many applications it is not ideal. Less error can now be tolerated in the received communication signal compared to an ideal modulated communication signal in the more modern communication systems. Less error can now be tolerated because power levels must be kept as low as possible to maintain transmissions within an assigned frequency band and higher modulation orders are being used to convey communications. Accordingly, if the receiver implements receiver-applied corrections to correct a transmitter imbalance, such receiver-applied corrections are applied to noise as well as signal. Consequently, such corrections tend to exaggerate the influence of noise in the received communication signal. Moreover, if quadrature imbalance at a transmitter is extreme, then a receiver-implemented correction may be utterly unsuccessful.
Another prior art solution to this quadrature imbalance problem relies on a separate, dedicated, special purpose receiver co-located with a transmitter to detect quadrature imbalance conditions at the transmitter and take corrective actions. While this solution does not face the problem of distinguishing between receiver imbalances and transmitter imbalances, this solution is highly undesirable due to the expense of the special purpose receiver to be co-located with the transmitter.
Accordingly, a need exists for a communication system that corrects for quadrature signal imbalance in a transmitter using the same system receiver that is used to extract data from a received communication signal.